The present invention relates to a heat sink capable of managing the heat from a heat source like an electronic component. More particularly, the present invention relates to a graphite-based heat sink system effective for dissipating the heat generated by an electronic component, wherein the heat sink includes a plurality of spaced parallel planar radial fin members.
With the development of more and more sophisticated electronic components, including those capable of increasing processing speeds and higher frequencies, having smaller size and more complicated power requirements, and exhibiting other technological advances, such as microprocessors and integrated circuits in electronic and electrical components and systems as well as in other devices such as high power optical devices, relatively extreme temperatures can be generated. However, microprocessors, integrated circuits and other sophisticated electronic components typically operate efficiently only under a certain range of threshold temperatures. The excessive heat generated during operation of these components can not only harm their own performance, but can also degrade the performance and reliability of the overall system and can even cause system failure. The increasingly wide range of environmental conditions, including temperature extremes, in which electronic systems are expected to operate, exacerbates the negative effects of excessive heat.
With the increased need for heat dissipation from microelectronic devices, thermal management becomes an increasingly important element of the design of electronic products. Both performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment. For instance, a reduction in the operating temperature of a device such as a typical silicon semiconductor can correspond to an exponential increase in the reliability and life expectancy of the device. Therefore, to maximize the life-span and reliability of a component, controlling the device operating temperature within the limits set by the designers is of paramount importance.
Heat sinks are components that facilitate heat dissipation from the surface of a heat source, such as a heat-generating electronic component, to a cooler environment, usually air. In many typical situations, heat transfer between the solid surface of the component and the air is the least efficient within the system, and the solid-air interface thus represents the greatest barrier for heat dissipation. A heat sink seeks to increase the heat transfer efficiency between the components and the ambient air primarily by increasing the surface area that is in direct contact with the air. This allows more heat to be dissipated and thus lowers the device operating temperature. The primary purpose of a heat sink is to help maintain the device temperature below the maximum allowable temperature specified by its designer/manufacturer.
Typically, heat sinks are formed of a metal, especially copper or aluminum, due to the ability of metals like copper to readily absorb heat and transfer it about its entire structure. In many applications, copper heat sinks are formed with fins or other structures to increase the surface area of the heat sink, with air being forced across or through the fins (such as by a fan) to effect heat dissipation from the electronic component, through the copper heat sink and then to the air.
Limitations exist, however, with the use of metallic heat sinks. One limitation relates to the relative isotropy of a metalxe2x80x94that is, the tendency of a metallic structure to distribute heat relatively evenly about the structure. The isotropy of a metal means that heat transmitted to a metallic heat sink becomes distributed about the structure rather than being directed to the fins where most efficient transfer to the air occurs. This can reduce the efficiency of heat dissipation using a metallic (e.g., copper) heat sink. Moreover, this relative isotropy is not readily controlled or varied, and provides no opportunity for preferentially directing heat.
In addition, the use of copper or aluminum heat sinks can present a problem because of the weight of the metal, particularly when the heating area is significantly smaller than that of the heat sink. For instance, pure copper weighs 8.96 grams per cubic centimeter (g/cc) and pure aluminum weighs 2.70 g/cc (compare with graphite articles, which typically weigh less than about 1.8 g/cc). In many applications, several heat sinks need to be arrayed on, e.g., a circuit board to dissipate heat from a variety of components on the board. If metallic heat sinks are employed, the sheer weight of the metal on the board can increase the chances of the board cracking or of other equally undesirable effects, and increases the weight of the component itself.
What is desired, therefore, is a heat sink system effective for dissipating heat from a heat source such as an electronic component. The heat sink system should advantageously be relatively anisotropic, as compared to a metal like copper or aluminum and exhibit a relatively high ratio of thermal conductivity to weight. One group of materials suitable for use in heat sinks are those materials known as graphites.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as xe2x80x9cflexible graphitexe2x80x9d). The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is as much as about 80 times or more the original xe2x80x9ccxe2x80x9d direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cc to about 2.0 g/cc. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the xe2x80x9ccxe2x80x9d direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the xe2x80x9caxe2x80x9d directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the xe2x80x9ccxe2x80x9d and xe2x80x9caxe2x80x9d directions.
The present invention provides a heat sink apparatus for an electronic component. The apparatus includes a base constructed from a graphite material. It includes a plurality of spaced parallel planar fin members supported by the base, each fin member including a planar fin of an anisotropic graphite material having graphene layers aligned primarily with the plane of the fin, so that each fin exhibits thermal conductivity in directions parallel to the plane of the fin substantially greater than thermal conductivity perpendicular to the plane of the fin.
The fins are preferably formed by pressing the fins from a sheet of anisotropic flexible graphite material, preferably resin-impregnated flexible graphite material. The fins may be pressed with an integral core portion, in which case the graphene layers of both the fin and the core are substantially aligned in planes generally parallel to the plane of the fins.
Alternatively, the fin members may be pressed with a circular center opening in which is received a separately formed core portion. The separately formed core portion may be formed such that its graphite layers are substantially aligned in planes generally perpendicular to the planes of the fins.
It is an object of the present invention is to provide a heat sink system exhibiting a relatively high degree of anisotropy.
Yet another object of the present invention is to provide a heat sink system having a relatively high ratio of thermal conductivity to weight.
Still another object of the present invention is to provide a heat sink system that can be fabricated so as to locate the heat dissipation surfaces thereof so as to control and/or maximize the dissipation of heat from the heat source.
Another object is to provide a radial finned heat sink having a stack of spaced radially extending fin plates, with the graphite layers of each fin plate substantially aligned in planes primarily parallel to the planes of the fin plates.
Yet another object of the present invention is the provision of a construction for a radial finned heat sink from graphite material, in which the core of the heat sink may be constructed to have the graphite layers thereof aligned primarily in planes parallel to a core axis and perpendicular to the planes of the fins.
Another object is to provide an economical construction for a radial finned heat sink constructed of graphite material.
Other and further objects, features, and advantages will be readily apparent to those skilled in the art, upon a reading of the following disclosure when taken in conjunction with the accompanying drawings.